The selfheal timer was downloading prerelease versions (e.g., v5.0.0-rc.1)
even when users had the stable channel selected. This happened because
fetch_latest_release_tag() didn't filter out prereleases from the GitHub
releases API response.
Now both the Python-based parser and the grep fallback skip prereleases:
- Python: checks `release.get("prerelease")` field
- Grep fallback: filters out -rc, -alpha, -beta patterns
Related to #849
3594fc692a